The micropore-mesopore composite molecular sieves having double grade pores of micropores and mesopores have incorporated the pore advantage of mesopore materials with the strong acidic property and the high hydrothermal stability of micropore molecular sieves and made both of the materials complementary and cooperative. Furthermore, the pore diameter and acidic property are both adjustable, that is, a composite material with different pore allocations and acidic property distributions can be prepared by performing an optimized combination of two selected materials with different pore structures and acidic properties. The successful preparation and versatile modes of the molecular sieves with multi-grade pores characterized in assembly will have a wide application perspective in more fields (Nature, 417 (2002) 813).
The synthesis of micropore-mesopore composite molecular sieves began with the preparation of MCM-41/FAU composite material reported by Kloets tra etc. (Micro. Meso. Mater. 6 (1996), 287). Subsequently, numerous researchers devoted to this research field and developed many new composite methods of micropore-mesopore composite molecular sieves. According to the difference of the structure characteristics, the micropore-mesopore composite molecular sieves have two types of composite modes: (1) a composite of two materials of micropore molecular sieve and mesopore molecular sieve. In this type of composite mode, the two materials of micropore molecular sieve and mesopore molecular sieve often show a coating structure, an embedding structure, or a complex combination of both of the structures. Here, an obvious connected interface (transition layer) is present between the two materials and the results characterized by X-ray diffraction (XRD) will show the diffraction peaks corresponding to the two materials, respectively; (2) a composite of mesopores and micropores in a molecular sieve material, and this type of composite mode includes two manners: a micropore molecular sieve with mesopores, that is, a micropore molecular sieve introduced with mesopores, contributes to the diffusion of molecules while maintaining the strong acidic property and stability of the micropore molecular sieve; a mesopore molecular sieve with parts of the properties of micropore molecular sieve, that is, a mesopore material whose amorphous pore walls have been introduced with a primary or secondary structure unit of zeolite, realizes pore walls in a nanometer range. The detailed research progress on this aspect is described in “Petrochemical Technology” (02 (2005) 188). Up till now, all the reported mesopore-micropore composite molecular sieves are of the molecular sieve system consisted of silicon and aluminum, however, as to the molecular sieve system consisted of silicon, phosphorus and aluminum, there are no opened literature reports because the synthesis system is relatively complex.
SAPO-34 molecular sieve was disclosed in U.S. Pat. No. 4,440,871 in 1984. According to the definition of IUPAC on pore diameter size, SAPO-34 belongs to the small pore molecular sieves (<2 nm). SAPO-34 molecular sieve has attracted attention because it has shown superior catalysis performance in the conversion reaction of methanol to olefins (MTO).